Sealed sapphire wave guide window



Dec. 4, 1956 F. BROOK 2,773,245

SEALED SAPPHIRE WAVE GUIDE wmnow Filed Oct. 16. 1952 25 24 2 6 7 I 22 2|1:9 20 E I IO M 25 2| l8 I9 26 2o 9 FIG 1 4 INVENTQR; FRqAIK WQOKFITTORNY United States Patent SEALED SAPPHIRE WAVE GUIDE WINDOW FrankBrook, Harrow, England, assignor to The General Electric CompanyLimited, London, England, a British company Application October 16,1952, Serial No. 315,113

Claims priority, application Great Britain October 19, 1951 3 Claims.(Cl. 333-98) This invention relates to electrical devices including ahollow metal tube arranged to act as a waveguide carrying high frequencyenergy in operation of the device and closed by the sealing to the tubeof a closure member of insulating material. The closure member usuallyconsists of a disc, which may in some cases be dished into the form of acup. The invention is especially, though not exclusively, concerned withsuch devices in which the gas pressures on the two sides of the saidclosure member are appreciably different, for example linear electronaccelerators, or magnetrons in which the tube forms part of the outputsection and the closure member separates the evacuated interior of themagnetron from the external atmosphere.

In such devices adapted to handle high power, we have found thatconsiderable heat is generated in the said closure member, and that ifthis member is of glass, which is usually preferable for ease andefficiency of sealing to the metal tube, it is liable to soften and bedestroyed, especially if there is a difference of gas pressure on thetwo sides of the member. The softening of the glass invariably takesplace at the centre of the closure member, the outer part of the memberbeing cooled by thermal conduction along the metal tube.

The object of the invention is to provide a convenient method ofreducing this difficulty.

According to the invention, in an electrical device of the kind referredto the said closure member of insulating material consists of a centralportion of relatively refractory material sealed to a less refractoryouter portion of glass which is in turn sealed to the metal tube.

The outer portion of glass may in some cases consist of two or morerings of glasses of different thermal expansion coefficients sealedtogether to form a graded seal matching on one side the said relativelyrefractory material and on the other side the metal of the tube.

The said relatively refractory material may consist, for example, ofceramic material but preferably it consists of a disc of sapphire whichmay conveniently be sealed to the glass outer portion by the followingmethod.

The disc of sapphire is supported on a block of refractory materialwhich does not readily oxidise (for example an iron-chromium-nickelalloy), with an annular washer of a suitable heat-resistive material(such as silver) intervening between the disc and the block, the inneredge of the washer extending everywhere slightly within the periphery ofthe disc and the outer edge of the washer projecting for an appreciabledistance beyond the periphery of the disc; the open end of a tube ofglass suitable for sealing to the disc and fitting closely around it isrested on the projecting edge of the washer, and the glass tube and discare heated by heating of the block so as to soften the end of the glasstube and cause it to flow on to and seal to the edge of the disc.

The tube and disc are removed from the block, with the washer adherentthereto, the washer is then stripped 2,773,246 Patented Dec. 4, 1956off, and the seal between the disc and the glass is further heated toproduce a smooth seal and for annealing. The end of the tube containingthe sapphire disc can then be cut off to any desired length.

One embodiment of the invention will be described, by way of example,with reference to Figure 1 and Figure 2 of the accompanying drawing,wherein:

Figure 1 shows a cross-sectional view along the longitudinal axis of amagnetron provided with a Wave-guide output section in accordance withthe invention, and

Figure 2 shows a perspective View of the wave guide output sectionsealed with a composite closure member in accordance with the invention.

In this embodiment the magnetron comprises a sealed evacuated envelope1, partially surrounded by a water cooling jacket 2 provided with inletand outlet pipes 3, 4 respectively for the passage of cooling water inoperation; the envelope 1 includes a substantially cylindrical anodeblock of conducting material, such as copper, recessed from each end toleave a central portion which forms the anode proper; this centralportion is perforated by a central hole 5 along the axis of the block,and within this hole, substantially at the centre thereof, there issupported a hollow cylindrical cathode within which is arranged a heater7 and which is provided with end discs in the usual manner.

For supporting the cathode, and making the necessary electricalconnections to it and the heater, the cathode is attached at each end toa stout lead-in-wire (one only of which appears in the Figure 1 view);the leadin-wires pass on either side of the anode through a hole in thewall 9 of the block, and through a pipe 10 forming a passage sealedthrough the water-jacket 2, each pipe 10 being closed at its outer end11 by means of a glass to metal seal 12, through which the lead 8passes.

A plurality of slots 13 are formed in the anode around the central hole5, extending radially from the hole, and each of the slots terminates ina further circular hole 14 formed through the block parallel with thecentral hole, the slots 13 and holes 14 constituting the anode cavityresonators in known manner.

One 15, of the cavity resonators is continued through the Wall of theanode block by a further radial slot 16, and this slot is continued by apair of exponentially curved lip plates 17, mounted one along each sideof the slot parallel to the axis of the block 1, so as to form awaveguide transformer for the passage of high frequency electricalenergy from the magnetron, in operation, into a waveguide outputsection.

The latter includes a copper tube 13 formed with a flange 19 at its endadjacent to the anode block, the flange being sealed to a mounting plate20 which is fixed to the water jacket 2 and sealed to the outer surfaceof the block around the edge of the slot 16, the other end of the tube18, remote from the block, is formed with a feather edge 21 around itsperiphery and is closed by a composite closure member in accordance withthe invention.

This composite closure member comprises a sapphire disc 22, sealed tothe inner periphery of a cup-shaped glass outer ring 23 by any suitablemethod, such, for example, as that previously described; the member issealed across the end of the copper tube 18 by means of a glass to metalseal formed between the outer edge of the ring of glass and the featheredge of the copper tube; a further copper tube 24 of larger internaldiameter than the external diameter of the tube 18 and formed at one endwith a flange 25 for attaching the tube to a further waveguide sectionis provided at the other end with a further flange 26 and is attached,by means of ice this flange, to the mounting plate 20, so that this tubesurrounds the tube 18.

In operation of this embodiment, when the magnetron is placed betweentwo suitable magnetic poles (not shown) for creating an axial magneticfield and the magnetron is supplied with the appropriate operatingcurrent and voltage, high frequency oscillations are set up within themagnetron which are transmitted through the exponentially curvedlip-plates 17 into the tube 18 and pass from this tube through theclosure member formed by the sapphire disc 22 and glass outer ring 23,into the further tube 24, which forms a further waveguide section.

When this high frequency energy passes through the closure member aconsiderable amount of heat is produced in it due to dielectric losses;the outer part of the closure member which is formed from glass is,however, cooled by thermal conduction along the metal tube and thecentre portion of the member, being formed from sapphire, is capable ofwithstanding considerably more heat than is ordinary glass, so that themagnetron can operate with a very high power output without damaging theclosure member.

I claim:

1. An electrical device including a hollow metal tube arranged'to act asa waveguide carrying high frequency energy in operation of the deviceand closed by a thin closure member of insulating material, said closuremember consisting of a central fiat sapphire disk and an annular glasspiece, the outer periphery of said annular glass piece being sealed tothe metal tube and the inner periphery of said annular glass piece beingsealed to the periphery of the sapphire disk.

2. A magnetron including a waveguide output section including a hollowmetal tube having an output end, and means closing said end, said meansconsisting of a central flat sapphire disk and an annular glass piece,the outer periphery of said annular glass piece being sealed to themetal tube and the inner periphery of said annular glass piece beingsealed to the periphery of the sapphire disk.

3. An electrical device according to claim 1 wherein the said outerportion of glass consists of two or more rings of glasses of difierentthermal expansion coefiicicnts sealed together to form -a graded sealmatching on one side the said refractory material and on the other sidethe metal of the tube.

References Cited in the file of this patent UNITED STATES PATENTS2,311,520 Clifford Feb. 16, 1943 2,576,186 Malter Nov. 27, 1951 FOREIGNPATENTS 766,374 France June 27, 1934 OTHER REFERENCES Reference Data forRadio Engineers, 3rd edition, Federal Telephone & Radio Corp.Publication, August 1949, pp. 48 and 49. Copy in Division 69.

